As part of a broad program to study aerobic energy metabolism in parasites and mammals, current interest is focused primarily on the role of iron-sulfur flavoproteins as electron carriers in the respiratory chain of Entamoeba histolytica. Flavins were detected and analyzed by difference and fluorescent spectrophotometry; oxygen consumption was determined polarographically. Axenic amebae contained 7% Riboflavin, 21% FAD and 72% FMN of the total TCA-extractable flavins. The flavins, like non-heme iron, were found in the soluble fraction of E. histolytica. Riboflavin kinase was detected also in the soluble fraction. Its specific activity (4.6 Units/mg protein) approximated that of rat liver. In contrast to known kinases, the amebal enzyme was activated by Mn 2 ion instead of Mg 2 ion. These findings show that E. histolytica has the intrinsic capacity to synthesize flavonucleotides. A collaborative study of electron probe microanalysis of cestode calcareous corpuscles, initiated this year, showed differences in the distribution of metals in the complex organic-mineral structure of these granules. The collaborative studies on bovine adrenal medulla chromaffin granules disclosed that although uncouplers of oxidative phosphorylation prevented the release reaction, presumably by discharging a proton gradient, anion channel blockers were ineffective as uncouplers, and must act by another mechanism. Catalase, but not peroxidase, was detected in the chromaffin granule fraction and may be involved in the synthesis of biogenic amines.